Simple question for you geometry gurus…

[Vortac]

So I have a hard tail with an ETT length of 575mm and seat angle of 74.5 degrees.  I intend to build a FS bike with an ETT length of also 575mm but with a seat angle of 78.3 degrees.

For seated position purposes only… how would this difference in SA of 3.8 deg affect riding position, if at all?  Main objective here is comfort while seated, my hard tail does the trick, trying to get as close as I can with new FS build.

Thanks!

[zilcho]

ETT being the same means that you should be able to use the same stem and bar setup to achieve the same reach on the bike, but the steeper ST will mean that you are farther forward in relation to the BB if you use the same saddle position. Depending on how dialed your position is at the moment this could lead to discomfort, or it could mean nothing. The steeper ST means that you should be over the rear wheel more on steeper climbs, in theory leading to better traction and less chance of loop out.

[veeTee1Pah]

I saw an interesting video of why you can't just replicate geometry numbers on a hard tail and full suspension and expect them to feel similar.


(Edit: Looks like my desktop browser doesn't embed the video well. Search "Are you riding the right size bike? | Bike Geometry Explained by Jeff Lenosky" if you can't see the embedded video.)

The youtuber, Jeff Lenosky, is a skilled biker and settled on a reach 40mm shorter on his hard tail for a similar feel to his full suspension bike after trial and error.

Geometry changes very differently through suspension travel on a hard tail versus a full suspension bike. Even at sag, the bikes with the same reach on the geometry charts have changed in different ways. On a full suspension, the "effective" reach shortens as the suspension squats. On a hard tail, the reach feels like it lengthens. Other geo numbers also probably change quite differently as the suspension squats.

[Vortac]

ETT being the same means that you should be able to use the same stem and bar setup to achieve the same reach on the bike, but the steeper ST will mean that you are farther forward in relation to the BB if you use the same saddle position. Depending on how dialed your position is at the moment this could lead to discomfort, or it could mean nothing. The steeper ST means that you should be over the rear wheel more on steeper climbs, in theory leading to better traction and less chance of loop out.

Good info, much appreciated!

[Vortac]

I saw an interesting video of why you can't just replicate geometry numbers on a hard tail and full suspension and expect them to feel similar.




The youtuber, Jeff Lenosky, is a skilled biker and settled on a reach 40mm shorter on his hard tail for a similar feel to his full suspension bike after trial and error.

Geometry changes very differently through suspension travel on a hard tail versus a full suspension bike. Even at sag, the bikes with the same reach on the geometry charts have changed in different ways. On a full suspension, the "effective" reach shortens as the suspension squats. On a hard tail, the reach feels like it lengthens. Other geo numbers also probably change quite differently as the suspension squats.

Interesting take, I’ll definitely check the video out…

I didnt  take into account sag too much since I thought that could be adjusted by saddle height, but didn’t consider new reach while  seated down at different saddle heights due to the Seat Angle…

Thx!

[veeTee1Pah]

Interesting take, I’ll definitely check the video out…

I didnt  take into account sag too much since I thought that could be adjusted by saddle height, but didn’t consider new reach while  seated down at different saddle heights due to the Seat Angle…

Thx!

It's not just the seat tube angle that changes. Even standing, the "effective reach" changes as the suspension travels depending on the type of suspension. I was surprised that a reach difference of 40mm made the bikes feel similar.

[Cerps]

If ETT is the same on both bikes they shouldn't feel any different when seated and stationary.  Effective top tube calculation accounts for differences in seat tube angle.  Effective top tube is the virtual distance from a horizontal line that runs between the top of the head tube and the imaginary line extending from the seat tube that it would intersect.  Think of it as the "seated reach" of the bike. 

And reach is the distance between a vertical line that runs through the bottom bracket and the top of the center of the head tube?  So this is the "standing reach" of the bike. 

Look at the Light Carbon 917 and 918.  The 917 has a shorter reach (421.2) but slightly longer top tube (600) because it's seat tube angle is 73.  The 918 has a 14mm longer reach (435mm) but shorter top tube (598.8MM) because it's seat tube angle is steeper (74.75). With same seat, stem, spacers, bar; these 2 bikes should feel very much the same when you sit on them. 

In your case things will be a little different because the FS will sag a little so I think puts your weight back a little more than the HT. But how far you have to stretch to hold the grips should still be the same.



 

[Vortac]

Thanks Cerps, it makes complete sense.  The light carbon examples you mention made it pretty easy to understand.  Thanks a million.

[acedeuce802]

https://madscientistmtb.com/bike-geometry-compare/

Input both bikes into this calculator.  To compare hardtail properly, input at the top a negative fork travel for how much sag you have.  Then in the bottom calculations, "Calculated Seated Fit" is saddle to bars.  You'll find that with the same ETT, your saddle to bars will be very close and it's the reach that differs.  So it'll feel similar length while seated, but your body is rotated forward with a bit more weight on the hands, but a steeper STA longer reach bike will feel longer while standing.

The picture visualization is perfect to see how they compare, then if you really like your current fit, you can see how stem, stack, handlebar, etc changes will dial it in, so you can make a good guess at what size components to start with.